1. Field of the Invention
The present invention relates generally to semiconductor devices and, more particularly, to a semiconductor device including a silicon thin film for use as an interconnection or an electrode. The invention further relates to a method of manufacturing such a semiconductor device.
2. Description of the Background Art
In order to apply a conductivity to a silicon thin film for use as an interconnection or an electrode, it is necessary to implant arsenic or phosphorus into the silicon thin film.
FIGS. 5A-5E show a method of forming an interconnection or an electrode by implanting arsenic or phosphorus into a silicon thin film.
With reference to FIG. 5A, a field oxide film 2 is formed on a surface of a silicon substrate 1.
A gate electrode 4 is formed on the silicon substrate 1 with a gate oxide film 3 interposed therebetween, with reference to FIG. 5B.
With reference to FIG. 5C, an interlayer insulating film 5 is formed on the silicon substrate 1 to cover the gate electrode 4. Openings 5a and 5b for forming a source region 6 and a drain region 7 are formed in the interlayer insulating film 5. After that, implanting impurity ions onto the surface of the silicon substrate 1 leads to formation of the source region 6 and the drain region 7. With reference to FIG. 5D, a silicon thin film 8 is formed to fill the openings 5a and 5b. The silicon thin film 8 does not exhibit a conductivity unless it includes impurities. It is thus necessary to implant arsenic or phosphorus. With phosphorus employed as impurity ions 10, since phosphorus has a higher diffusion coefficient, there arises a problem that phosphorus goes through the silicon thin film 8 and arrives in the silicon substrate 1. When arsenic is employed as the impurity ions 10, since arsenic has a lower diffusion coefficient, the above problem does not arise, however, the impurity ions are likely to remain in the surface of the silicon thin film 8. Thus, there is a problem that arsenic is not implanted at a uniform concentration along the depth of the silicon thin film 8.
With reference to FIG. 5E, the silicon thin film implanted with the impurity ions is patterned to form an interconnection 11 (or an electrode).
Formation of an interconnection by employing an impurity ion implantation method has the above-described problems.
As a method of implanting arsenic into a silicon thin film at a uniform concentration along the depth of the thin film, a CVD method as described below has been developed. (Journal of Electronic Materials, Vol. 19, No. 12, 1990, pp. 1395-1402)
FIG. 6 is a conceptual diagram of a CVD apparatus for use in formation of a silicon thin film to which arsenic is added, on semiconductor wafer 14. This CVD apparatus includes a vacuum chamber 12. The vacuum chamber 12 has a diameter of e.g., 30 cm and a length of e.g., 2 m. For example, 100 sheets of silicon wafer 14 are disposed in the vacuum chamber 12. A heater 13 is provided outside the vacuum chamber 12. The vacuum chamber 12 incorporates a silane source 15 for introducing silane (SiH.sub.4) into the vacuum chamber 12 and an arsenic source 16 for introducing an arsine gas (AsH.sub.3) into the vacuum chamber 12.
The silane gas and then the arsine gas are introduced into the vacuum chamber 12, and the inside of the vacuum chamber 12 is heated to 600.degree.-800.degree. C. by use of the heater 13. This heating decomposes the silane into silicon atoms and hydrogen, and the arsine into arsenic atoms and hydrogen. With the produced silicon atoms and arsenic atoms deposited on the semiconductor wafer 14, a silicon thin film including arsenic is formed.
As described above, arsine is employed in the formation of the silicon thin film by employing the conventional CVD method. This arsine is extremely highly toxic as well known. It is reported in general that a lethal dose of arsine is 0.05 ppm. Thus, a conventional step of forming a silicon thin film with addition of arsenic, using arsine is one of the most dangerous works in a semiconductor manufacture process.